This project is a part of interinstitutional cooperative study to determine the effectiveness of oral administration of acetylsalicylic acid (aspirin) in the reduction or prevention of transient cerebral ischemic attacks and amaurosis fugax. In addition, to study the effect on the frequency of cerebral and myocardial infarction in long-term follow-up. This is a controlled clinical trial in which only individuals having the hemispheric type of transient ischemic attack or monocular blindness will be included. Those with primary vertebro-basilar insufficiency only, or with potentially fatal complicating medical conditions, will be excluded. Care will be taken to exclude individuals found to have blood dyscrasia or specific idiosyncracy to aspirin. Subjects are allocated to subgroups on the basis of a clinical decision to operate or not to operate. In each of the two subgroups, subjects will be assigned to treatment with aspirin or a placebo on the basis of a random statistical design. Strict surveillance will be maintained of the patient's adherence to the instructions for therapy. The administration of the active drug and placebo is on a coded, double blind basis. A mechanism is provided for in the project design for breaking the code in the event of clinical necessity. Nine of twenty-five patients with hemispheric attacks answered the criteria of the study and were entered from October 1, 1972 to June 1, 1973.